DC-DC converters convert input DC voltage to output DC voltage to drive a load. Buck type DC-DC converters include an inductor and one or more switches to provide output voltages up to the input voltage level. Boost converters also include an inductor and one or more switches to provide output voltages in excess of the input voltage. Buck-boost DC-DC converters can generate output voltages below or above the input voltage level.
FIG. 10 shows an inductive inverting buck-boost DC-DC converter 1000 with first and second (high side and low side) transistors M1 and M2 connected in series between an input voltage VIN and an output voltage VOUT. An inductor L is connected between a center switch node SW joining the transistors M1 and M2 and a ground connection. An input capacitor CIN is connected between VIN and ground, and an output capacitor COUT is connected between the output voltage VOUT and ground. In operation, the first transistor M1 is turned on while M2 is turned off in a first portion of a switching cycle for current flow along a first path 1001, and M2 is turned on while M1 is turned off in a second portion of a switching cycle for current flow along a second path 1002. During the first portion of each switching cycle, M1 is turned on which connects the switch node SW to the supply voltage VIN. The inductor voltage is therefore VIN which results in a rising inductor current. In the first switching cycle portion, moreover, the voltage across M2 is VIN+|VOUT|. During the second switching cycle portion, M2 is turned on and M1 is turned off to connect the switch node SW to the output voltage VOUT. In this condition, the voltage across the inductor L is −VOUT, resulting in a falling inductor current, and the voltage across M1 is VIN+|VOUT|. Switching operation of M1 and M2 in this fashion provides a negative output voltage VOUT with an absolute value that can be greater than, less than, or equal to the level of the input voltage VIN.
The transistors M1 and M2 in the inverting buck-boost converter 1000 in FIG. 10 are exposed to the sum of the magnitudes of input voltage VIN and output voltage VOUT. In addition, the transistors M1 and M2 of the converter 1000 must be sized to withstand at least VIN +|VOUT| in switching mode, the converter 1000 requires transistors with higher voltage ratings than with standard buck or boost converters. The most severe situation comes during switching when M1 and M2 operate in the saturation region and conduct the full inductor current while blocking a high source-drain voltage. Increasing the blocking voltage of the transistors M1 and M2 increases the device size and thus the switching losses are increased compared with conventional buck converters. In addition, the voltage swing on the switch node SW and thus the voltage across the inductor L is higher than the inductor voltage swings associated with a conventional buck converter or a conventional boost converter. Although buck-boost topologies offer a wider output voltage range for a fixed input voltage VIN, the buck-boost converter 1000 suffers from a larger ripple current and higher inductive switching losses compared with buck or boost converter architectures. As a result, the conventional buck-boost converter 1000 cannot achieve efficiencies possible with a buck converter or with a boost converter.
FIG. 11 shows a three level converter 1100 using two additional transistors M3 and M4 connected between M1 and M2, and a capacitor C connected to additional capacitor nodes CAP1 and CAP2 across M3 and M4 to reduce the effective transistor and inductor voltage. The steady state capacitor bias voltage is (|VO|+VI)/2 and is subtracted from the input voltage VIN during a first switching cycle portion with M1 and M4 turned on for conduction along a path 1101. During the second switching cycle portion with M2 and M3 turned on, the capacitor voltage is subtracted from the output voltage VOUT and the voltage across the switches M1-M4 is reduced compared with the buck-boost converter 1000 of FIG. 10. In addition, the switch node voltage is reduced, resulting in lower inductor current ripple and lower conduction losses. However, the converter circuit 1100 of FIG. 11 has many disadvantages. In each switching cycle portion, two of the four transistors are connected in series, and the total switching loss is significant even though the individual transistors M1-M4 are smaller than transistors used in the buck-boost converter 1000 of FIG. 10. In addition, the converter 1100 includes two extra transistors M3 and M4, and three nodes that must be switched during each switching cycle. Furthermore, the capacitor C is typically too large to be integrated on a single chip, and thus two additional pins are needed for access to the CAP1 and CAP2 nodes. Moreover, there is no inherent reset state for the capacitor bias voltage, and an extra control loop is needed to avoid capacitor voltage run-away conditions.